1. Field of the Invention
The present invention relates to a slider having an improved profile. It is used in the reading and/or writing of information on a magnetic support and in particular on hard disks.
2. Description of the Prior Art
FIGS. 1 to 8 relate to the prior art in connection with sliders and more particularly provide a basis for the description of the technical problem associated with the take-off of sliders and their behavior in operation.
FIG. 1 very diagrammatically shows the structure of a slider 10, which has on the face to be placed facing the recording support, two rails or skis 12, 14. A magnetic read and/or write head 16 is placed at the rear of the head. This head can be placed on the edge of the slider (not shown) or on the lower face of one of the sliders. Moreover, there is a chamfer 18 at the front of each ski.
FIG. 2 shows such a slider in the inoperative position on the recording support 20, which is assumed to be a hard disk. When the disk is inoperative, the slider is in contact therewith.
When the disk starts to rotate, the slider rubs on the disk. As soon as the speed reaches a certain threshold (called take-off speed), a tongue of air rushes under the chamfer 8 and consequently creates a lifting force, which brings about the take-off of the slider. This is illustrated in FIG. 3, where the air flow is designated 22.
When the disk rotation speed reaches its nominal level, the slider is in dynamic equilibrium between the lifting force acting on the chamfer and the skis and also a bearing force (indicated by the arrow 24) caused, on the other face of the slider, by a spring (not shown) and which applies a force of 3 to 5 g to the slider.
The angle .alpha. formed by the slider with the disk must be as large as possible at the time of take-off in order to ensure rapid head take-off and prevent longer rubbing of the head on the disk. However, if the angle .alpha. is too large, the flight or movement stability of the head decreases. Therefore a compromise must be found between these opposing requirements and in practice the angle .alpha. is fixed at a few microradians.
Moreover, prior to take-off, a slider is located above the internal tracks of the disk, i.e. on the disk center side, where the tangential speed of the disk is lowest (for a given angular speed). Therefore take-off occurs at low speeds.
It is therefore appropriate to give the chamfer 18 a considerable size in order to ensure a rapid take-off of the head. However, when the head moves towards the median or external tracks of the disk, the tangential speed increases, which has the effect of raising by the same amount the slider, thus increasing the angle .alpha. and therefore making head flight unstable.
FIG. 4 shows the evolution of the angle .alpha. as a function of the radius r of the overflown track. The angle .alpha..sub.1, obtained for the external radius Rext, is the maximum angle which can be accepted for a correct flight or movement of the head. The angle .alpha.0 is the angle obtained for the internal radius Rint. If the chamfer had been optimized to obtain the value .alpha.1 for the internal radius, there would certainly have been a rapid head take-off for the internal radius, but as soon as the head moved towards the external tracks, the angle .alpha. would have reached values no longer complying with standards and the slider stability would have decreased.
In order to obtain a necessary lift at take-off, it must be noted that a chamfer at the front of the skis is not the only solution. It is also possible to machine a step or indentation, which is often simpler.
Thus, FIG. 5 diagrammatically shows two embodiments of the slider, i.e. with a chamfer 18, or with a step or stair 19. Such sliders are described in numerous documents, e.g. U.S. Pat. No. 4,673,996 and European patent 543 690.
The presence of a step at the front of the skis has the effect of creating an overpressure beneath the ski and which is located very close to the step. This is illustrated by FIG. 6, where the bottom diagram shows the overpressure P along the longitudinal axis of the ski and the top diagram the position of the slider. It can be seen that the over-pressure passes through a maximum 21 level with the riser 23 of the front indentation.
In order to optimize the efficiency of the step, its height (i.e. the height of the riser) should be adapted to the relative speed between the disk and the slider at which it is wished to create the overpressure. Thus, at a low speed, e.g. approximately 3 m/s, the step height will be approximately 100 nm. For a speed of about 10 m/s, the step height will be approximately 800 nm.
In the frequently encountered case where a slider must take-off in an internal track of a disk, where the speed is low, a step of limited height will make it possible to create the overpressure appropriate for take-off. However, when the disk speed has reached its nominal level, the limited height step will lose its effectiveness. A step having a greater height would have been more effective.
It is for this reason that it has been proposed to provide the front part of the skis with two steps instead of one. This is shown in FIG. 7, where there is a first step 30 with its riser 31 and a second step 32 with its riser 33. These two steps ensure both a correct take-off above the internal tracks and an appropriate inclination above the external tracks.
A slider having two steps is described in U.S. Pat. No. 3,488,648.